Formation of prostheses from polytetrafluoroethylene (PTFE), particularly expanded polytetrafluoroethylene (ePTFE) is well known in the prior art. ePTFE includes a node and fibril structure, having longitudinally extending fibrils interconnected by transverse nodes. The nodes are not particularly strong in shear, and, thus, ePTFE structures are susceptible to failure in a direction parallel to the fibril orientation. ePTFE structures (tubes, sheets) are typically paste extruded, and, the fibrils are oriented in the extrusion direction.
Vascular grafts formed of ePTFE are well known in the art. Where sutures have been used to fix such grafts, suture hole elongation and propagation of tear lines from suture holes have been noted.
To overcome the deficiencies of the prior art, techniques have been developed which re-orient the node and fibril structure of an ePTFE element to be transverse to the extrusion direction. By orienting the fibrils at an angle relative to the extrusion direction, the tear strength of a respective product may be greatly improved. In one technique set forth in U.S. Pat. Nos. 5,505,887 and 5,874,032, both to Zdrahala et al., an extrusion machine is described having a counter-rotating die and mandrel arrangement. Accordingly, upon being extruded, a single-layer unitary PTFE tube is formed having an outer surface twisted in one helical direction, and an inner surface twisted in an opposite helical direction. Although tubes formed in accordance with the method of U.S. Pat. Nos. 5,505,887 and 5,874,032 are expandable to form an ePTFE structure, the fibrils of the structure are oriented generally parallel to the expansion direction after expanding as shown in the micrograph of FIG. 5 in U.S. Pat. No. 5,874,032. Also, the tube tends to thin out unevenly under expansion, and, suffers from “necking”.